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I know this thread is kind of older...but I also had some similar issues a while back when I lived in DC...they treated the water with orthophosphate...which also acts as a buffer, like baking soda (calcium carbonate) does...therefore, I would get high pH readings (close to 8) with a low KH reading. I would add baking soda in a pinch, but eventually switched to Buff it up from goldfish connection and even added crushed coral to the filter...problem was...that my KH still wouldn't budge...

The issue i believe was because orthophosphate also acts as a buffer, there was nothing to "buff" (ie no free hydrogen ions for my buffer additives to attach too)...but my test kit (API drops) only account for the calcium carbonate as a buffer...the crushed coral didn't work because there was no reason for it to dissolve in solution as the orthophosphate was acting as the buffer instead...therefore any additive I would use would just fall out of solution and look like dirt at the bottom of my tank (and become dried white deposits elsewhere)...the only thing that worked on the KH was the baking soda...but that isn't stable long term...

My solution was move to a different part of the country (haha), but otherwise I would have tried RO water/tap water mix to get my aqueous calcium levels up by adding Buff it Up to the RO water....and to mix it with the tap water

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I know this thread is kind of older...but I also had some similar issues a while back when I lived in DC...they treated the water with orthophosphate...which also acts as a buffer, like baking soda (calcium carbonate) does...therefore, I would get high pH readings (close to 8) with a low KH reading. I would add baking soda in a pinch, but eventually switched to Buff it up from goldfish connection and even added crushed coral to the filter...problem was...that my KH still wouldn't budge...

The issue i believe was because orthophosphate also acts as a buffer, there was nothing to "buff" (ie no free hydrogen ions for my buffer additives to attach too)...but my test kit (API drops) only account for the calcium carbonate as a buffer...the crushed coral didn't work because there was no reason for it to dissolve in solution as the orthophosphate was acting as the buffer instead...therefore any additive I would use would just fall out of solution and look like dirt at the bottom of my tank (and become dried white deposits elsewhere)...the only thing that worked on the KH was the baking soda...but that isn't stable long term...

My solution was move to a different part of the country (haha), but otherwise I would have tried RO water/tap water mix to get my aqueous calcium levels up by adding Buff it Up to the RO water....and to mix it with the tap water

The phosphate buffering system is also a very good buffering system, and you are right, it does not register on the kH test, because there is no carbonate/bicarbonate involved, which is what the kH tests measure.

Besides from buffering the water, having a good kH is important, because carbonates are sources of carbon for your beneficial bacteria.